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Homework Help: How come water in a cup won't shoot out of a straw placed inside it?

  1. Apr 27, 2014 #1
    I've been learning about pressure and fluid dynamics, and I've stumbled onto a problem. Say you have a straw in a cup. The surface area of the water in the cup is much greater than that of the straw. So I would expect Atm. Pressure × Large Area gives a much greater force than Atm. Pressure × Small Area. In fact, the force is so much larger I would expect a spectacular jet of water, which obviously does not happen.

    What am I missing?

  2. jcsd
  3. Apr 27, 2014 #2


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    What's the pressure inside an open straw with one end put into a cup of water? Why doesn't the water shoot out of the cup without a straw placed in it?
  4. Apr 27, 2014 #3
    The water doesn't shoot out because the atmosphere is pressing down on it evenly. And the pressures are the same, but the forces are different. I know by experience that the water won't shoot out of the straw, but I don't know why in terms of physics models.

    I know you're trying to hint me to the answer but I don't really understand.

  5. Apr 27, 2014 #4


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    Try to specify clearly all of the separate forces acting on the column of water inside the straw and the locations at which those forces act. What is the magnitude of each of the forces?

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  6. Apr 27, 2014 #5
    Why are you comparing the forces? You should be comparing the pressures. The pressure inside the straw is the same as the pressure outside of the straw (for the same depth) therefore there is no net force pushing fluid inside of the straw. The atmospheric force is not transferred unchanged over the fluid. The pressure is. That's called Pascal principle.
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